Bone anchored hearing aids are essential for the rehabilitation of patients suffering from some specific type of hearing losses for which traditional hearing aids are insufficient. This type of device consists of an external hearing aid with a vibrating transducer which is connected via a coupling to a skin penetrating abutment mounted on a fixture anchored in the skull bone. It is important that the coupling is sufficiently firm to avoid poor transmission of the vibrations but it is also important that the coupling is not too firm since it is also important that the hearing aid falls off in case of a sudden impact to avoid that the skull bone anchoring is damaged. In a coupling like this there is always coupling forces pressing components in the connection against the abutment. The coupling forces can be generated by a separate spring, a flexible material or by a magnet. The patient takes on and off the hearing aid daily so wear and tear durability of the coupling is also important.
Patents such as U.S. Pat. No. 5,735,790, U.S. Pat. No. 4,498,461 SE 89032718 and SE0102207 describe couplings where the force when connecting and disconnecting the hearing aid is quite high since it is important that the hearing aid is firmly coupled to the abutment. Especially it is not possible for the patient to use the hearing aid at all during the time it takes for the fixture in the skull bone to integrate with the skull bone, which takes around 3–6 months. The patients need to come to the hospital three times, first for the fixture insertion, then after a week to remove a healing cap and some gauze around the abutment and then after 3–6 months for the hearing aid fitting. If any of the couplings in U.S. Pat. No. 5,735,790, U.S. Pat. No. 4,498,461 SE 89032718 or SE0102207 would be used by the patient during the healing time, the fixture would most likely not integrate with the skull bone and come loose, due to the high load on the fixture when connecting and disconnecting the hearing aid. Arrangements, like the one described in SE503790, have several disadvantages and are not a solution to this since they do not at all reduce the force when connecting the hearing aid and when disconnecting the devices a significant rotation force is generated which is not desirable since the fixture in the bone is often screw shaped. A rotation of the fixture would definitely hinder the integration in the bone.
The high load on the connection also limits the lifetime of the hearing aid. In the hearing aid especially the vibrating transducer and the transducer suspension are affected by and worn out by the high forces when taking the hearing aid on and off. A limitation of the current systems is also that it is not possible to do more powerful devices since especially for high frequencies there is distortion of the sound due to the limited coupling force available in the current designs. If the coupling force could be significantly higher than in the current systems also more powerful devices for patients with more severe hearing losses could be rehabilitated with this type of hearing aid. In spite of the fact that bone anchored hearing aids have been available since more than 15 years ago, the above mentioned drawbacks remain. There is a need to provide a solution to the above outlined problems and drawbacks.